Avoid needless memory slot in allocator

[kanigsson]

The allocator currently systematically allocates a memory slot for Write statements that involve messages, but such a slot is not always needed. I will refine the condition to allocate the slot to represent the need of the session generator.

[treiher]

The special handling of Write statements will most probably be obsolete after implementation of #807. Write (as well as Read statements) will have to be in a separate state and refer to a global variable. Using a message aggregate inside a Write statement will not be supported anymore. So we can defer this issue until #807 is merged and then remove the extra allocation for Write statements completely.

[kanigsson]

We could do it that way, but this change here is beneficial for my changes on #713, so waiting on #807 would create an unneeded dependency. However, I'm OK with that if you think that this commit would lead to more issues/conflicts with #807.

[treiher]

Never mind, I did not see that you already created a PR for that. I just thought we could prevent unnecessary work, but as it is already fixed, we can also merge it now.